A clock source is a basic required building block of any digital System on Chip (SoC). For example, a low frequency clock may be utilized as a real time clock (RTC) for the circuit or as an input reference clock. The device which utilizes such an SoC solution is quite often battery operated, and in this ultra-low power (ULP) application environment it is critical that the clock source possess an inherent low power consumption as well as an operation that is insensitive to process, voltage and temperature (PVT) variation.
There are a number of well-known solutions for implementing ULP clock sources. One conventional approach utilizes a quartz oscillator (XTAL) because such a clock source has a very stable frequency and low deviation over time. However, this XTAL solution has a high area overhead, generally exhibits a slow starting time/power tradeoff characteristic, and cannot be fully integrated. Another solution uses complementary metal oxide semiconductor (CMOS) circuits for LC resonators or relaxation circuits based on switching capacitors which advantageously support full integration and frequency stability over PVT variation, but operate with increased power consumption as well as increased component counts and sizes. Another solution is an ULP ring oscillator (RO), but the inherent structures of the circuit tend to be highly sensitive to PVT variation. On-chip oscillators using leakage currents have also been proposed, but the accuracy of the timer is not very well controlled across the fabrication process thus necessitating a need for an accompanying calibration circuit.